1. Field of the Invention
The invention relates to a ring-shaped resistance heater for supplying heat to a growing single crystal.
2. Background Art
One method for producing single crystals is the Czochralski method. It is employed, for example, for producing single crystals composed of silicon. In accordance with this method, firstly a melt is produced in a crucible, then a seed crystal is brought into contact with the melt and, finally, a single crystal suspended from the seed crystal is caused to grow by the seed crystal being pulled upward from the surface of the melt.
It is advantageous if heat can be supplied to the growing single crystal by means of a ring-shaped resistance heater, which is also called a ring heater hereinafter. Thus, US 2004/0192015 describes a method wherein heat is supplied to the phase boundary of the growing single crystal by means of a ring heater in order to homogenize the axial temperature gradients at the phase boundary along the radius of the single crystal. The aim of the measure is to influence the formation of intrinsic point defects to the effect that the defect properties of the grown single crystal are as uniform as possible in a radial direction.
U.S. Pat. No. 6,132,507 mentions that a ring heater can also be used to reduce fluctuations of the diameter of the single crystal and the growth rate.
The inventors of the present invention have ascertained that the use of a ring heater for supplying heat to a growing single crystal poses difficulties if, at the same time, a strong external magnetic field is built up in order to influence the flow in the melt. An electromagnetic interaction between the external magnetic field and the magnetic field generated by the ring heater occurs in such a case. The Lorentz force acting on the ring heater can alter the position of the ring heater, damage the ring heater or even destroy the latter.
U.S. Pat. No. 7,335,864 B2 describes the fact that the inherent magnetic field of a resistance heater can be reduced by a heating coil being wound in a bifilar fashion, such that the electric current flows in opposite directions through the partial turns of the heating coil. However, despite the use of a ring heater modified in this way, the difficulties mentioned cannot be eliminated satisfactorily because the interaction between the magnetic fields remains, albeit in weakened form.